I agree "has not been found" is fair. I wonder if anyone has ever tried to do a comprehensive survey of known scripts.
Perhaps someone on the old mailing list did - but I've seen no mention of it.
I expect Stolfi looked at some Asian scripts, and recently Thomas Spande introduced the question of Armenian script and language. The Friedmans invited a couple of Egyptologists, I gather. I've mentioned Sabaic minuscule and Greco-Bactrian..
It would be an interesting research project. And in this, I keep thinking of a manuscript which was mentioned by Okasha el Daly. It is said to have a large number of scripts recorded in it, each with their equivalent sound in Arabic. It is said to be in the Topkapi serai, so if anyone feels like visiting during their hols...